In Situ X-Ray Diffraction Study of Phase Transformations in Nanostructured Ti-Nb-(Zr,Ta) SMA Under Variable Stress-Temperature Conditions

Metastable near-β titanium alloys are among the most promising candidates for the role of metallic implant materials because they could be designed to manifest low Young’s modulus and superelastic behavior which closely mimic that of bone. These alloys are nickel-free, in contrast to Ti-Ni shape memory alloys (SMA), and therefore entirely biocompatible. Ti-Nb-based SMA are multiphase materials with multiple solid-state phase transformations, however, only one of them, a reversible  thermoelastic β↔α” martensitic transformation, leads to shape memory and superelasticity effects realization. To allow better understanding of the phase transformation phenomena in these alloys under variable temperature and stress conditions, this work is focused on their in situ X-ray diffraction analysis in comparison with the results of macroscopic thermomechanical testing. This analysis was performed on Ti-Nb-Ta and Ti-Nb-Zr biomedical SMA with a nanosubgrained β-phase microstructure obtained as a result of thermomechanical processing implying cold rolling and post-deformation annealing. A custom-designed tensile stage lodged within a TTK450 low-temperature thermal chamber of a “PANalytical X’Pert PRO” diffractometer was used for experiments under three testing sequences: (1) stress-free temperature scanning (reference sequence); (2) constant-strain temperature scanning with a specimen loaded in the β-austenitic state and (3) constant-strain temperature scanning with a specimen loaded in the mixed β-austenite + α”-martensite state. The tensile stage, which functioning is based on an independent two-way shape-memory Ti-Ni actuator, is described. Based on the results obtained, analysis of the temperature- and strain-induced α”- and ω-phases formation and their reverse transformations in both Ti-Nb-based SMA is presented. The α”-martensite and β-austenite lattice parameters and their temperature- and stress-induced variations, as well as β↔α” transformation lattice strains, are calculated.